1. Field of the Invention
The present invention relates to an optical disc apparatus having a tracking control function for causing an optical beam to accurately follow an eccentricity of a track of a rotating disc-shaped information carrier (hereafter referred to as an xe2x80x9coptical discxe2x80x9d) having tracks each comprising a pit or a guide groove.
2. Related art of the Invention
To reproduce a signal, a conventional optical disc apparatus irradiates an optical disc with relatively faint optical beams of a fixed light quantity to detect reflected light modulated for intensity by the optical disc. To record a signal, the device modulates the light quantity of optical beams depending on a recorded signal to write information to a recording material film on the optical disc (for example, Japanese Patent Laid-Open No. 52-80802).
A reproduction-only optical disc has pit-based information spirally recorded therein. A recordable and reproducible optical disc is produced by using evaporation or the like to form an optically recordable and reproducible material film on a base material surface having spiral tracks of a relief structure. Recording information on the optical disc or reproducing the recorded information requires focus control for controlling optical beams in a perpendicular direction (hereafter referred to as a xe2x80x9cfocus directionxe2x80x9d) of the optical disc surface in such a manner that the optical beams are always in a predetermined converging state on the recording material film, and tracking control for controlling the optical beams in a radial direction (hereafter referred to as a xe2x80x9ctracking directionxe2x80x9d) so that the beams are always located on a predetermined track.
A retrieval operation by the conventional optical disc will be described with reference to FIG. 16. An optical head 10 has a semiconductor laser 11, a coupling lens 12, a polarizing beam splitter 13, a quarter wavelength plate 14, a focus actuator 16, a tracking actuator 17, a detection lens 18, a cylindrical lens 19, and a photodetector 20 mounted therein. Optical beams generated by the semiconductor laser 11 are made parallel beams by the coupling lens 12, so that the parallel beams pass through the polarizing beam splitter 13 and the quarter wavelength plate 14 and are converged on a disc-shaped optical disc 1 by a converging lens 15.
The thus reflected beams pass through the converging lens 15 and the quarter wavelength plate 14, are subsequently reflected by the polarizing beam splitter 13, and then pass through the detection lens 18 and the cylindrical lens 19 onto the photodetector 20, which is divided into four. The converging lens 15 is supported by an elastomer and is electromagnetically moved in the focus direction when current flow through the focus actuator 16 or in the tracking direction when current flows through the tracking actuator 17. The photodetector 20 transmits a detected light quantity signal to a focus error generator 30 (hereafter referred to as an xe2x80x9cFE generator 30xe2x80x9d) or a tracking error generator 40 (hereafter referred to as a xe2x80x9cTE generator 40xe2x80x9d).
The FE generator 30 uses the light quantity signal from the photodetector 20 to calculate an error signal (hereafter referred to as an xe2x80x9cFE signalxe2x80x9d) indicating how the optical beams are converging on an information surface of the optical disc 1 and transmits the signal to the focus actuator 16 via a focusing filter 31 (hereafter referred to as an xe2x80x9cFc filter 31xe2x80x9d). The focus actuator 16 controls the converging lens 15 in the focus direction so that the optical beams converge on a recording surface of the optical disc 1 in a predetermined state. This is the focus control.
The TE generator 40 uses the light quantity signal from the photodetector 20 to calculate an error signal (hereafter referred to as a xe2x80x9cTE signalxe2x80x9d) indicating the positional relationship between the optical beams and tracks on the optical disc 1 and then transmits the signal to the tracking actuator 17 via a tracking filter 41 (hereafter referred to as a xe2x80x9cTk filter 41xe2x80x9d) and an adder 42. The tracking actuator 17 controls the converging lens 15 in the tracking direction in such a manner that the optical beams follow the tracks. This is the tracking control.
A driving signal from the Tk filter 41 is transmitted to an memory 60. A motor 50 rotates the optical disc 1 to transmit 1,000 encoder pulses per one rotation to a rotation phase detect device 51. The rotation phase detect device 51 counts rising edges in the encoder pulses from the motor 50, and clears the count value to zero when 1,000 pulses corresponding to one rotation are counted, thereby to transmit the resulting rotation phase information to the memory 60. An eccentricity compensation controling signal 62 transmits a control signal to the memory 60.
If the control signal from the eccentricity compensation controling signal 62 is at a high level, the memory 60 stores a signal from the Tk filter 41 at an address corresponding to rotation phase information from the rotation phase detect device 51, and continues transmitting zero to a lowpass filter 61. If the control signal from the eccentricity compensation controling signal 62 is at a low level, the memory 60 transmits a value stored at an address that is based on the rotation phase information from the rotation phase detect device 51, to the adder 42 via the lowpass filter 61. The adder 42 adds a signal from the Tk filter 41 and a signal from the lowpass filter 61 together and then transmits the resulting signal to the tracking actuator 17.
The operation will be explained with reference to FIG. 17. FIG. 17a shows rotation phase information from the rotation phase detect device 51, FIG. 17b shows a control signal from the eccentricity compensation controling signal 62, and FIG. 17c shows a signal from the lowpass filter 61. In FIG. 17, before t0, eccentricity correction is not working, between t0 and t1, eccentricity correction learning is being executed, and after t1, the eccentricity correction is working. Between t0 and t1, the memory 60 stores the signal from the Tk filter 41 at the address that is based on the rotation phase information as shown in FIG. 17a. 
At the time t1, the memory 60 completes storing eccentricity correction driving for one rotation of the optical disc. After t1, the memory 60 outputs the stored eccentricity correction driving and passes it through the lowpass filter 61 to obtain a driving waveform such as that shown in FIG. 17c. Since a driving waveform for causing to following the eccentricity shown in FIG. 17c is applied in addition to the perpendicular tracking control, the tracking actuator 17 accurately follows the eccentricity of the tracks.
Not only the signal from the Tk filter 41 in the tracking control state but also the TE generator 40 in a tracking non-control state may be used for measurements for generating the eccentricity correction driving (Japanese Patent Laid Open No. 3-272030).
Eccentricity is caused by the deviation of a rotation center or track waves; it is mostly caused by the deviation of the rotation center. The components of the rotation center deviation comprises only the rotation frequency components of the optical disc.
With the tracking control, eccentricity correction corrects a main component of the rotation center deviation to enable the optical beams to accurately follow the tracks. When a signal waveform is to be recorded in the memory 60, the result of the recording is affected by noise or the like. As shown in the left figure in FIG. 18a, the waveform generally includes components other than the rotation frequency components of the optical disc, so that the result of the recording in the memory 60 has an error with respect to a target sine wave as shown in the right figure in FIG. 18a. To eliminate such an error, there has been a method for recording waveforms for two or more rotations for averaging in the memory 60. However, this method disadvantageously requires a large amount of time for measurements.
Alternatively, mostly without the tracking control, a deviation in the tracking direction caused by the eccentricity correction driving has a predetermined gain and a predetermined phase delay due to a transfer characteristic of the tracking actuator 17. Accordingly, the deviation in the tracking direction of the converging lens 15 caused by the driving for correcting the eccentricity has an error with respect to eccentricity. To suppress the correction error, there has been a method for modifying the eccentricity correction driving by measuring the deviation of the lens 15 from the eccentricity correction driving in the tracking direction using a sensor for sensing the absolute position of the lens 15 in the tracking direction (Japanese Patent Laid Open No. 2-141809). Due to the use of sensors, however, this measure disadvantageously requires a large number of parts and high costs.
Additionally, the rotation phase information from the rotation phase detect device 51 is generated using a rotation start time as a reference. If no access has occurred for a long time in order to reduce power consumption and once the optical disc apparatus has entered a sleep state, the power to the motor 50 and the rotation phase detect device 51 also remains off during this period to change the reference for the rotation phase information output from the rotation phase detect device 51. Disadvantageously, once the reference for the rotation phase information from the rotation phase detect device 51 has changed after the sleep state, the eccentricity correction driving adjusted before the sleep state can no longer be used and must be readjusted due to a change in phase. There is such problem.
To solve the above problems, the present invention comprises a TE signal detecting means of detecting a positional error between an optical beam and a track of an optical disc, a tracking control means of controlling the optical beam so as to follow the track based on a TE signal from the TE signal detecting means, a rotation number measuring means of measuring a rotation number of the optical disc, an eccentricity measuring means of measuring the magnitude and direction of eccentricity relative to a rotation phase of the optical disc based on a signal from the TE signal detecting means and the measured rotation number from the rotation number measuring means, and an eccentricity correcting means of generating tracking driving based on the magnitude and direction of the eccentricity from the eccentricity measuring means.
Additionally, the present invention comprises aTE signal detecting means of detecting a positional error between an optical beam and a track of an optical disc, a tracking control means of controlling the optical beam so as to follow the track based on a TE signal from the TE signal detecting means, a rotation number measuring means of measuring a rotation number of the optical disc, an eccentricity measuring means of measuring the magnitude and direction of eccentricity relative to a rotation phase of the optical disc based on a driving signal from the tracking control means and the measured rotation number from the rotation number measuring means, and an eccentricity correcting means of generating tracking driving based on the magnitude and direction of the eccentricity from the eccentricity measuring means.
Further, the present invention comprises a TE signal detecting means of detecting a positional error between an optical beam and a track of an optical disc, a tracking control means of controlling the optical beam so as to follow the track based on a TE signal from the TE signal detecting means, an address detecting means of detecting the address of an information carrier irradiated with the optical beam, and a rotation phase measuring means of determining a reference for a rotation phase from the detected address to measure the rotation phase of the optical disc.
Furthermore, the present invention comprises a TE signal detecting means of detecting a positional error between an optical beam and a track of an optical disc, a tracking control means of controlling the optical beam so as to follow the track based on a TE signal from the TE signal detecting means, a switching point detecting means of detecting a switching point from a recess to a projection of a track groove of the optical disc or from the projection to the recess thereof, and a rotation phase measuring means of determining a reference for a rotation phase from the detected switching point position to measure the rotation phase of the optical disc.
Moreover, the present invention comprises a TE signal detecting means of detecting a positional error between an optical beam and a track of an optical disc, a tracking control means of controlling the optical beam so as to follow the track based on a TE signal from the TE signal detecting means, a disturbance detecting means of measuring a disturbance in rotation phase information on the optical disc, and a rotation phase measuring means of determining a reference for a rotation phase from the detected disturbance of the rotation phase to measure the rotation phase of the optical disc.